This invention relates to a plasma X-ray source which creates a plasma of high temperature and high density by using impulse discharge to generate soft X-rays, and more particularly to an improvement in an X-ray source suitable for, for example, an X-ray aligner adapted to manufacture sub-micron integrated circuits.
Among various existing types of plasma X-ray sources, a plasma X-ray source such as a plasma focus type or a gas puff and pinch type source utilizing a discharge of a gas at a lower pressure than atmospheric pressure is especially suited for the X-ray source for X-ray aligner because it has high X-ray generation efficiency and the lifetime of its electrodes is long.
In a plasma focus type source which has been studied and reported in detail by J. W. Mather in Methods of Experimental Physics, Vol. 9, PtB, page 189 (1971, Academic Press), two coaxial cylindrical electrodes, which are, at one end, electrically insulated from each other by means of a glass tube and are opened at the other end, are disposed inside a discharge chamber which is filled with a gas, the gas is ionized by applying across the electrodes an impulse voltage produced from a charged capacitor to create an initial plasma, and the initial plasma is moved in a space between the electrodes to form an ultimate plasma of high temperature and high density which is focused on the tip of one electrode serving as a target and which is used for generation of neutrons and soft X-rays from the ultimate plasma.
The plasma focus type source is advantageous in that its construction and operation can be simplified for generation of strong soft X-rays but suffers from problems that reproducibility is poor for each cycle of discharge and repetition of discharge is slow. These problems are inherent to such a discharge tube as the plasma focus type source based on filling of a working gas and subsequent generation of dicharge. Especially, the problem of poor reproducibility is due to the fact that under discharging, atoms of impurity gases absorbed in the electrodes are emitted into the discharge space with the result that the working gas is mixed with the impurity gases and contaminated thereby, causing the pressure in the discharge tube to charge. It follows therefore that the discharge condition is so changed as to impair the reproducibility. To prevent this problem, there needs a series of operations of evacuating the working gas after completion of each cycle of discharge and re-filling the working gas at a predetermined pressure. For evacuation and re-filling of the working gas, a commensurate time is consumed which determines a period for each cycle of repetitions discharge. The throughput of wafers is of significance in the X-ray aligner and the time consuming plasma focus type source is unsatisfactory for use with the X-ray aligner. In addition, because of the series of evacuation and re-filling operations, surface conditions of the insulating members and the electrodes accommodated in the discharge tube change to a great extent, giving rise to possible impariment of the stationary operation. Further, the discharge tube has an unused space which does not participate in light emitting or discharging. The entire working gas inclusive of even a portion thereof prevailing in the unused space is exchanged and therefore the amount of gas used is increased inevitably.